This invention relates generally to a system and method for providing a motion-compensated drilling rig platform. More particularly, the invention relates to a system and method which can be used to control marine riser disconnection events in conjunction with such a platform.
Drilling operations conducted from a floating vessel require a flexible tensioning system which operates to secure the riser conductor between the ocean floor (at the well head) and the rig, or vessel. The tensioning system acts to reduce or eliminate the affects of vessel heave with respect to the riser, and to mitigate the effects of planned riser disconnect operations, and unexpected breaks or faults in the riser (hereinafter a xe2x80x9cdisconnect eventxe2x80x9d).
Riser tensioner devices, which form the heart of the tensioning system, have been designed to assist in the management of riser conductors attached to drilling rigs, especially with respect to movement caused by periodic vessel heave. A series of these tensioners, connected to the riser by corresponding cables and sheaves, react to relative movement between the ocean floor and the vessel by adjusting the cable length to maintain a relatively constant tension on the riser. Any number of tensioners, typically deployed in pairs, may be used to suspend a single riser from the vessel.
Unexpected events may occur during offshore drilling operations. These may occur in the form of tensioner wireline breaks, severe storms, or other circumstances which require the vessel/rig operator to act quickly to adjust the tension applied to the riser. The riser may also become disconnected from the wellhead for various reasons.
The need to rapidly disconnect the riser as a planned operation, or the need to respond to an unexpected riser disconnect event, and manage the recoil tension or xe2x80x9cslingshotxe2x80x9d effect on the vessel induced thereby provides the motivation to develop a system and method to control the movement of the disconnected riser under tension. The system and method should operate by managing the tension applied to the riser using cabling attached to the riser and a plurality of riser tensioners. The system and method should also operate in response to sensing a disconnect event (typically provided by a Lower Marine Riser Package (LMRP) sensor), or in response to a discrete, operator-supplied, command which prepares the system to anticipate a riser disconnect. Thus, the system and method should be simple, robust, and provide an intermediate level of operation (i.e. xe2x80x9carmed and ready to sense/manage a riser disconnect eventxe2x80x9d), such that system elements are demonstrated to be properly connected, and yet, not actively managing a disconnect event.
In one embodiment, the riser recoil control system of the present invention adjusts a series of tension forces applied to a marine riser, which is in turn typically attached to an anchored, floating vessel. The riser can be connected to, and disconnected from, a wellhead, and is attached to the vessel using tension forces exerted by a plurality of riser tensioners connected to the riser with cables and sheaves, and mounted to the vessel. Each tensioner has an air shutoff valve, and an orifice-controlled fluid valve set to a preselected flow limit value. The tensioners may also include a fluid volume speed control valve which acts to limit the volumetric rate of fluid flow in the tensioner whenever the flow rate exceeds a predetermined, critical, volumetric rate of flow.
The system also includes a disconnection sensing means, such as a switch (e.g., a LMRP sensor), which provides a disconnect signal when the riser is disconnected from the wellhead. Application of the disconnect signal to the air shutoff valves and orifice-controlled fluid valves results in closing the valves and adjusting the tension forces applied to the marine riser by the tensioners so as to limit the rate of travel experienced by the tensioner pistons as the tension force on the riser is reduced over the course of a managed disconnect event. While the air shutoff valve is typically set to close completely upon sensing a disconnect event, the orifice-controlled fluid valve is typically set to close down to about 15% of the maximum value after disconnect.
The system may include a first timer which delays closure of the air shutoff valves, and a second timer which delays closure of the orifice-controlled fluid valves, after the disconnect signal is applied. The delay times may be selected to manage the xe2x80x9cslingshotxe2x80x9d effect of the disconnected riser upon the vessel.
The system may also include a manual arming means, such as an emergency disconnect switch on the BOP (Blowout Preventer) Control Panel, adapted to provide an arming signal upon activation by a human operator. This action alerts the system to anticipate and act upon a disconnect signal from the BOP stack plates as they separate (i.e., from the LMRP sensor). The received disconnect signal then triggers operation of the system in a similar fashion to that described above.
The invention also includes a method for adjusting the tension forces applied to the riser. Assuming the existence of a riser recoil control system constructed in a similar fashion to that just described, the method may comprise the steps of sensing the disconnect signal provided by the disconnection sensing means and adjusting the tension force applied to the riser by closing the plurality of air shutoff valves, and partially closing the orifice-controlled fluid valves so as to move them from a first xe2x80x9cpre-disconnectxe2x80x9d preselected value (of about 50% of maximum free-flow rate permitted by the valve) to a second xe2x80x9cpost-disconnectxe2x80x9d preselected flow rate value (preferably about 15% of the maximum free-flow rate value).